Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “equilibrium potential”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Computer simulation of surface and film processes

All the investigations which were performed employed in one way or another a computer simulation technique based on atomistic level considerations. In general, three types of simulation methods were used for modeling systems with discrete particles that interact via well defined potential functions: molecular dynamics (a general method for solving the classical equations of motion of a model system); Monte Carlo (the use of Markov chain ensemble averaging technique to model equilibrium properties of a system); and molecular statics (provides properties of a system at T = 0 K). The effects of three-body forces on the vibrational frequencies of triatomic cluster were investigated. The multilayer relaxation phenomena for low index planes of an fcc crystal was analyzed also as a function of the three-body interactions. Various surface properties for Si and SiC system were calculated. Results obtained from static simulation calculations for slip formation were presented. The more elaborate molecular dynamics calculations on the propagation of cracks in two-dimensional systems were outlined.

Tiller, W. A.↗
NASA NTRS1991

Oxygen production by pyrolysis of lunar regolith

Oxygen was identified as the most important product of initial lunar materials processing efforts. A source of oxygen on the Moon provides an alternative to the costly transport of propellant to the Moon or to low earth orbit. Pyrolysis, or vapor-phase reduction, involves heating a feedstock to temperatures sufficient to decompose the constituent metal oxides and release oxygen. The process relies on the vaporization of metal oxides in the form of reduced suboxides or atomic species. The reduced species must then be condensed without re-oxidizing, yielding oxygen in the gas phase. The feasibility of obtaining oxygen from common lunar minerals was demonstrated using solar furnace experiments. These results are discussed together with chemical equilibrium models which were extended to include the multicomponent oxides used in experiments. For the first time, both experiments and theoretical models dealt with the complex oxides that make up potential lunar feedstocks. Two major conclusions are drawn from this preliminary work. First, unbeneficiated regolith is a suitable feedstock for pyrolysis. Second, the process can operate at moderate temperatures, circa 2000 K, which could be supplied by direct solar or electrical energy. In addition to these advantages in choice of feedstock and energy source, the pyrolysis process requires no chemicals or reagents, making it an attractive process for lunar oxygen production.

Senior, Constance L.↗
NASA NTRS2007

An Investigation of High-Cycle Fatigue Models for Metallic Structures Exhibiting Snap-Through Response

A study is undertaken to develop a methodology for determining the suitability of various high-cycle fatigue models for metallic structures subjected to combined thermal-acoustic loadings. Two features of this problem differentiate it from the fatigue of structures subject to acoustic loading alone. Potentially large mean stresses associated with the thermally pre- and post-buckled states require models capable of handling those conditions. Snap-through motion between multiple post-buckled equilibrium positions introduces very high alternating stress. The thermal-acoustic time history response of a clamped aluminum beam structure with geometric and material nonlinearities is determined via numerical simulation. A cumulative damage model is employed using a rainflow cycle counting scheme and fatigue estimates are made for 2024-T3 aluminum using various non-zero mean fatigue models, including Walker, Morrow, Morrow with true fracture strength, and MMPDS. A baseline zero-mean model is additionally considered. It is shown that for this material, the Walker model produces the most conservative fatigue estimates when the stress response has a tensile mean introduced by geometric nonlinearity, but remains in the linear elastic range. However, when the loading level is sufficiently high to produce plasticity, the response becomes more fully reversed and the baseline, Morrow, and Morrow with true fracture strength models produce the most conservative fatigue estimates.

Przekop, Adam↗
NASA NTRS1981

Quantum chemical calculation of the equilibrium structures of small metal atom clusters

The application of ab initio quantum mechanical approaches in the study of metal atom clusters requires simplifying techniques that do not compromise the reliability of the calculations. Various aspects of the implementation of the effective core potential (ECP) technique for the removal of the metal atom core electrons from the calculation were examined. The ECP molecular integral formulae were modified to bring out the shell characteristics as a first step towards fulfilling the increasing need to speed up the computation of the ECP integrals. Work on the relationships among the derivatives of the molecular integrals that extends some of the techniques pioneered by Komornicki for the calculation of the gradients of the electronic energy was completed and a formulation of the ECP approach that quite naturally unifies the various state-of-the-art "shape- and Hamiltonian-consistent" techniques was discovered.

Kahn, L. R.↗
NASA NTRS1984

Numerical simulation of reconnection in an emerging magnetic flux region

Numerical solutions in two dimensions are presented for the resistive MHD equations of an initial boundary value problem, simulating reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. This numerical model displays four main phases, which are interpreted in terms of (1) a slowly evolving quasi-steady phase during which most of the magnetic flux emerges, with approximate equilibrium between magnetic and pressure forces; (2) an impulsive phase in which either the reconnection or continuing emergence of the first phase comes to disrupt the equilibrium, and extensive acceleration occurs as the high pressure region within the emerging region drives the fluid upwards and outwards; (3) a second quasi-steady phase; and (4) a potential-static phase in which continuing reconnection, ohmic dissipation, and fluid transport through boundaries depletes the system of all currents and flows.

Forbes, T. G.↗
NASA NTRS1993

Large eddy simulation and direct numerical simulation of high speed turbulent reacting flows

The objective of this research is to make use of Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) for the computational analyses of high speed reacting flows. Our efforts in the first phase of this research conducted within the past three years have been directed in several issues pertaining to intricate physics of turbulent reacting flows. In our previous 5 semi-annual reports submitted to NASA LaRC, as well as several technical papers in archival journals, the results of our investigations have been fully described. In this progress report which is different in format as compared to our previous documents, we focus only on the issue of LES. The reason for doing so is that LES is the primary issue of interest to our Technical Monitor and that our other findings were needed to support the activities conducted under this prime issue. The outcomes of our related investigations, nevertheless, are included in the appendices accompanying this report. The relevance of the materials in these appendices are, therefore, discussed only briefly within the body of the report. Here, results are presented of a priori and a posterior analyses for validity assessments of assumed Probability Density Function (PDF) methods as potential subgrid scale (SGS) closures for LES of turbulent reacting flows. Simple non-premixed reacting systems involving an isothermal reaction of the type A + B yields Products under both chemical equilibrium and non-equilibrium conditions are considered. A priori analyses are conducted of a homogeneous box flow, and a spatially developing planar mixing layer to investigate the performance of the Pearson Family of PDF's as SGS models. A posteriori analyses are conducted of the mixing layer using a hybrid one-equation Smagorinsky/PDF SGS closure. The Smagorinsky closure augmented by the solution of the subgrid turbulent kinetic energy (TKE) equation is employed to account for hydrodynamic fluctuations, and the PDF is employed for modeling the effects of scalar fluctuations. The implementation of the model requires the knowledge of the local values of the first two SGS moments. These are provided by additional modeled transport equations. In both a priori and a posteriori analyses, the predicted results are appraised by comparison with subgrid averaged results generated by DNS. Based on these results, the paths to be followed in future investigations are identified.

Adumitroaie, V.↗
NASA NTRS1983

Kinetic effects on trace element partitioning

A potential difficulty regarding the application of trace element abundance data to problems in igneous petrogenesis is related to the possibility that partitioning of trace elements among minerals and melt may not be an equilibrium process. It may rather be controlled by kinetic processes, specifically the interplay between crystal growth and diffusion of the trace elements in crystals and melt. The present investigation has the objective to quantify kinetic effects on trace element partitioning. Crystals of olivine, pyroxene, and plagioclase were grown rapidly from synthetic melts resembling naturally-occurring mafic melts and doped with La, Sm, and Lu to quantify the magnitude of the effect of crystal growth rate on concentration of excluded trace elements. Olivine melt partitioning failed to show any appreciable kinetic effects, while diopside/melt distribution coefficients increased by 70 percent when the crystals grew very rapidly.

Lindstrom, D. J.↗
NASA NTRS1989

Three-dimensional equilibria for the extended magnetotail and the generation of field-aligned current sheets

Using the magnetotail equilibrium theory and a solution method outlined by Birn (1987), self-consistent three-dimensional models were constructed for the quiet average magnetotail beyond about 20 RE distance but earthward of a potential distant neutral line, which take into account the decrease of the tail flaring with distance. It is found that this effect is coupled with the presence of magnetic shear and thus with field-aligned electric currents. These currents have the signature of region 1 currents, toward the earth on the dawnside and away on the duskside, and contribute about 500,000 A to the total Birkeland current.

Birn, J.↗
NASA NTRS1991

Theory and simulation of a high-frequency magnetic drift wave

The equilibrium of a nonneutral plasma in a toroidal vessel with a toroidal magnetic field is analyzed. In the zero inertia limit it is heuristically shown from force balance considerations that there is an electrostatic hoop force and a force due to diamagnetism along the major radius. The problem of equilibrium is formulated in terms of solutions of a 2D partial difference equation. This equation is solved in the large-aspect-ratio limit and a general expression for the shift of the potential axis is obtained which shows that the shift is approximately epsilon and that it depends solely on the internal capacitance of the cloud. The simulation study is based upon the modified MHD equations and the nonlocal nature of the mode is investigated. Applications to sub-Alfvenic plasma expansions, electromagnetic waves in the earth's magnetosphere, and plasma switches are discussed.

Huba, J. D.↗
NASA NTRS1993

Program for an improved hypersonic temperature-sensing probe

Under a NASA Dryden-sponsored contract in the mid 1960s, temperatures of up to 2200 C were successfully measured using a fluid oscillator. The current program, although limited in scope, explores the problem areas which must be solved if this technique is to be extended to 10,000 R. The potential for measuring extremely high temperatures, using fluid oscillator techniques, stems from the fact that the measuring element is the fluid itself. The containing structure of the oscillator need not be brought to equilibrium temperature with with the fluid for temperature measurement, provided that a suitable calibration can be arranged. This program concentrated on review of high-temperature material developments since the original program was completed. Other areas of limited study included related pressure instrumentation requirements, dissociation, rarefied gas effects, and analysis of sensor time response.

Reilly, Richard J.↗
NASA NTRSDate not supplied

Adding GPU Support to the Markov Chain Monte Carlo Code Catmip

In geophysics, we are confronted with many under-determined inverse problems. For example, all of our observations of earthquakes are made at the Earth’s surface. So, when we try to infer how slip during an earthquake evolves in space and time, we find that there are many potential slip histories that are consistent with our limited observations and our understanding of earthquake physics. One way to approach these problems is with Bayesian analysis which allows us to infer the ensemble of all potential slip models that satisfy the observations and our prior knowledge of earthquake physics. In Bayesian analysis, our prior knowledge is known as the prior probability density function or prior PDF, the fit to the data is known as the data likelihood, and the target PDF that satisfies both the prior PDF and data likelihood is known as the posterior PDF. However, simulating the posterior PDF typically requires using Markov Chain Monte Carlo (MCMC) to draw tens of billions of random realizations of earthquake slip models, which may not be computationally feasible. To make this and similar geophysical inversions computationally tractable, we developed the Cascading Adaptive Transitional Metropolis In Parallel (CATMIP) algorithm. CATMIP is an efficient parallel Markov Chain Monte Carlo (MCMC) sampler that is used for model fitting and uncertainty quantification in geophysics. Example use cases are earthquake rupture modeling, determining mineral composition on Mars, reconstructing the history of ocean salinity, and historical earthquake relocation. CATMIP employs many parallel instances of the Metropolis algorithm for sampling in a transitioning framework. Transitioning is a process in which a set of random samples at equilibrium with a known probability density function (PDF) are used as seeds for the Markov chains to sample successive target PDFs that incrementally move the distribution from the starting seeds to the final desired PDF that describes the relative plausibility of potential values for the model parameters. The algorithm is implemented as a Master-Worker model employing MPI for communication. The worker processes are loosely coupled with global parameters periodically optimized by the master process. This provides a very high amount of parallelism with little communication between updates. During the presentation we will discuss the history of the algorithm and elaborate the earthquake rupture modeling use case for the CATMIP package. Our first step toward GPU optimization was to optimize the code for the CPU. CPU profiling revealed that most of the compute time is spent in calls to level 2 BLAS routines and calls to GSL random number generators. We revised the algorithm to employ level 3 BLAS routines instead. In our presentation we will describe how this was accomplished. Adding GPU support to CATMIP consisted mostly of replacing the calls to GSL with calls to GPU vendor-provided library routines. A small number of loops were directly implemented in CUDA. In the presentation will provide implementation details. Finally, we will discuss methods for profiling and opportunities for further optimizing GPU execution. By creating a code with the flexibility to run on either a CPU or GPU architecture, CATMIP can be used on systems ranging from large CPU-based HPC environments to single servers with GPU acceleration and everything in between.

HECC↗
NASA NTRS1986

Meridional distribution of stratospheric trace constituents

Vertically stratified tracers such as methane and nitrous oxide tend to have constant mixing ratio surfaces that slope downward toward the poles in the meridional plane. The equilibrium tracer slope results from the competition between the slope steepening effects of advection by the diabatic circulation and the slope flattening effects of quasi-isentropic eddy transport and photochemical loss. The diabatic circulation iself is, however, driven primarily by eddy transports, which maintain the departure of stratospheric temperatures from radiative equilibrium. If the eddy transports are weak, the diabatic circulation is also weak and the slope is small. Using a simple beta-plane channel model and an eddy diffusion parameterization for the eddy potential vorticity and tracer transports, it is shown that the slope is a maximum for a value of eddy diffusion such that the dynamical time scale lies between the radiative and chemical time scales.

Holton, J. R.↗
NASA NTRS1991

Coronal current-sheet formation - The effect of asymmetric and symmetric shears

A 2.5D numerical code is used to investigate the results of an asymmetric shear imposed on a potential quadrupolar magnetic field under two sets of atmospheric boundary conditions - a low-beta plasma with line tying at the base, similar to the line-tied analytic model, and a hydrostatic-equilibrium atmosphere with solar gravity, typical of the observed photosphere-chromosphere interface. The low-beta simulation confirms the crucial role of the line-tying assumption in producting current sheets. The effects of a symmetric shear on the same hydrostatic-equilibrium atmosphere is examined, using more grid points to improve the resolution of the current structures which form along the flux surfaces. It is found that true current sheets do not form in the corona when a more realistic model is considered. The amount of Ohmic dissipation in the thick currents is estimated to be two to four orders of magnitude below that required to heat the corona. It is concluded that magnetic topologies of the type examined here do not contribute significantly to coronal heating.

Karpen, Judith T.↗
NASA NTRS2009

A Theory for Rapid Charging Events on the International Space Station

The Floating Potential Measurement Unit (FPMU) has detected high negative amplitude rapid charging events (RCEs) on the International Space Station (ISS) at the morning terminator. These events are larger and more rapid than the ISS morning charging events first seen by the Floating Potential Probe (FPP) on ISS in 2001. In this paper, we describe a theory for the RCEs that further elucidates the nature of spacecraft charging in low Earth orbit (LEO) in a non-equilibrium situation. The model accounts for all essential aspects of the newly discovered phenomenon, and is amenable to testing on-orbit. Predictions of the model for the amplitude of the ISS RCEs for the full set of ISS solar arrays and for the coming solar cycle are given, and the results of modeling by the Environments WorkBench (EWB) are compared to the observed events to show that the phenomenon can be explained by solar array driven charging. The situation is unique because the coverglasses have not yet reached equilibrium with the surrounding plasma during the RCEs. Finally, a prescription for further use of the ISS for investigating fundamental plasma physics in LEO is given. Already, plasma and charging monitoring instruments on ISS have taught us much about spacecraft interactions with the dense LEO plasma, and we expect they will continue to yield more valuable science when the Japanese Experiment Module (JEM) is in place.

Ferguson, Dale C.↗
NASA NTRS2012

Lunar Impact Basins: Stratigraphy, Sequence and Ages from Superposed Impact Crater Populations Measured from Lunar Orbiter Laser Altimeter (LOLA) Data

Impact basin formation is a fundamental process in the evolution of the Moon and records the history of impactors in the early solar system. In order to assess the stratigraphy, sequence, and ages of impact basins and the impactor population as a function of time, we have used topography from the Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter (LRO) to measure the superposed impact crater size-frequency distributions for 30 lunar basins (D ≥ 300 km). These data generally support the widely used Wilhelms sequence of lunar basins, although we find significantly higher densities of superposed craters on many lunar basins than derived by Wilhelms (50% higher densities). Our data also provide new insight into the timing of the transition between distinct crater populations characteristic of ancient and young lunar terrains. The transition from a lunar impact flux dominated by Population 1 to Population 2 occurred before the mid-Nectarian. This is before the end of the period of rapid cratering, and potentially before the end of the hypothesized Late Heavy Bombardment. LOLA-derived crater densities also suggest that many Pre-Nectarian basins, such as South Pole-Aitken, have been cratered to saturation equilibrium. Finally, both crater counts and stratigraphic observations based on LOLA data are applicable to specific basin stratigraphic problems of interest; for example, using these data, we suggest that Serenitatis is older than Nectaris, and Humboldtianum is younger than Crisium. Sample return missions to specific basins can anchor these measurements to a Pre-Imbrian absolute chronology.

Fassett, C. I.↗
NASA NTRSDate not supplied

Silicon Partitioning Between Iron-Rich Metal and Silicate Melt: New Constraints From Aerodynamic Laser Levitation Furnace Experiments

The partitioning of light elements (e.g., Si, C, H, etc.) between silicate melts and iron metal alloys as a function of pressure and temperature is a critical constraint in defining the compositional evolution of planetary interiors. While there are a significant number of experimental studies in the literature that examine the various influences of oxygen fugacity, temperature, pressure, and composition on the metal-melt partitioning of silicon, it is difficult to disentangle the effects of each variable due to the nominally independent parameters (e.g., pressure, temperature, composition) being collinear. To better test for the independent effects of pressure, temperature, and melt composition on the partitioning of silicon between iron alloy and silicate melt, new high temperature (1800°–2500°C), rapid quench (~700°C/s) aerodynamic levitation laser-heating experiments in the Fe-Si-Mg-O (En65-Fa5-Si3-Fe27) system were conducted. This high temperature, low pressure method is ideal for these experiments because it eliminates the metal container as a sink for elements of interest to partition into, and it has a rapid quench rate that minimizes potential exsolution of silicon from the metal phase during quench cooling. Given the short run times of the levitation experiments (30–60 seconds), a time series at a lower temperature (1700°C) in a vertical gas-mixing muffle tube furnace was also conducted to assess equilibrium times of the bulk composition. All successful runs were imaged using back-scatter electron imaging and analyzed for phase composition using a JEOL 8530F field emission microprobe. These new data were added to a comprehensive and internally consistent data set created from published experiments, and subsequently modeled using parameters designed to minimize collinearity (e.g., optical basicity is used as a proxy for melt composition). High collinearity is found in the data set however between temperature and oxygen fugacity, precluding any model that contains both as nominally independent variables. The resulting model demonstrates a statistically significant effect of pressure, temperature, and melt composition on the partitioning of silicon in Fe-rich metal, which has implications for the formation of planetary cores in rocky planets and provides important constraints on potential bulk core compositions.

Metal↗
NASA NTRS1993

Isomerization reaction dynamics and equilibrium at the liquid-vapor interface of water. A molecular-dynamics study

The gauche-trans isomerization reaction of 1,2-dichloroethane at the liquid-vapor interface of water is studied using molecular-dynamics computer simulations. The solvent bulk and surface effects on the torsional potential of mean force and on barrier recrossing dynamics are computed. The isomerization reaction involves a large change in the electric dipole moment, and as a result the trans/gauche ratio is considerably affected by the transition from the bulk solvent to the surface. Reactive flux correlation function calculations of the reaction rate reveal that deviation from the transition-state theory due to barrier recrossing is greater at the surface than in the bulk water. This suggests that the system exhibits non-Rice-Ramsperger-Kassel-Marcus behavior due to the weak solvent-solute coupling at the water liquid-vapor interface.

Benjamin, Ilan↗